Method and apparatus for active pyrometry

Abstract

The present invention constitutes a pyrometer device and an associated method of operation for measuring temperature based on the radiation emitted by a heated body in which increased accuracy is achieved by actively ascertaining the emittance of the body whose temperature is being measured. The pyrometer device includes a light source for intermittently illuminating the heated body and a radiation sensing mechanism for measuring the amount of light reflected and radiated by the body. The pyrometer device further includes a signal processing unit for processing the information developed by the radiation sensing mechanism and deriving the temperature of the body based on a calculated emittance factor and the amount of light radiated by the body.

I claim: 
     
       1. A method of temperature measurement for use in semiconductor wafer fabrication processes comprising the steps of: (a) intermittently illuminating at a first frequency a semiconductor wafer with light from a light source;   (b) intermittently collecting at a second frequency light from the semiconductor wafer;   (c) measuring the amount of light reflected by said wafer when it is illuminated by said source as a function of the intermittently collected light;   (d) measuring the amount of light radiated by said wafer when it is not illuminated by said source as a function of the intermittently collected light;   (e) deriving the emittance of said wafer from the values measured in steps (c) and (d); and   (f) using said emittance in combination with the value measured in step (d) to derive the temperature of said wafer.   
     
     
       2. The method of claim 1 wherein steps (c) and (d) are performed within a band centered between 5.0 and 0.8 microns in wavelength. 
     
     
       3. The method of claim 1, wherein said step of illuminating said wafer includes conducting light from said source to said wafer using a fiber optic cable. 
     
     
       4. The method of claim 1 wherein steps (c) and (d) include filtering the light with a bandpass filter adapted for passing light centered at 1.46 microns in wavelength. 
     
     
       5. The method of claim 1 comprising providing a detector at a predetermined location relative to the wafer for collecting light received from the direction of the wafer, and wherein step (d) comprises the steps of (i) measuring the intensity of light received from said direction when the wafer is not illuminated by said source, (ii) measuring the intensity of light received from said direction when the light from the wafer is obscured from the detector, and (iii) subtracting the value measured in step (ii) from the value measured in step (i). 
     
     
       6. The method of claim 1 comprising providing a detector at a predetermined location relative to the wafer for collecting light received from the direction of the wafer, and wherein step (c) comprises the steps of (i) measuring the intensity of light received from said direction when the wafer is illuminated by said source, (ii) measuring the intensity of light received from said direction when the wafer is not illuminated by said source, and (iii) subtracting the value measured in step (ii) from the value measured in step (i). 
     
     
       7. The method of claim 1 wherein step (e) comprises deriving the reflectance of the wafer from the values measured in steps (c) and (d) and calculating the emittance of the wafer from the derived value of the reflectance. 
     
     
       8. The method of claim 7, comprising deriving the reflectance of the wafer by comparing the amount of light reflected by the wafer with the amount of light reflected by a body of known reflectance. 
     
     
       9. The method of claim 1 wherein step (d) comprises measuring the sum of the light radiated by the wafer plus background luminous flux, measuring the background luminous flux when light from the wafer is obscured, and subtracting the background luminous flux from the sum of the light radiated by the wafer plus background luminous flux. 
     
     
       10. A pyrometer for measuring temperature based on the radiation emitted by a heated body comprising: means for illuminating said heated body with light intermittently at a first frequency;   means for collecting light from said heated body intermittently at a second frequency and measuring both the amount of light reflected by said heated body and the amount of light radiated by said heated body as a function of the collected light;   means for deriving the emittance of said heated body based on the amount of light reflected by said body; and   means for deriving the temperature of said heated body based on said emittance and the amount of light radiated by said heated body.   
     
     
       11. The pyrometer of claim 10, wherein said means for illuminating said heated body includes; an infrared light source, and   a fiber optic means for conducting light from said source to said heated body.   
     
     
       12. The method of claim 11 wherein step (c) comprises measuring the amount of light reflected and radiated from the wafer and subtracting the amount of light radiated by the wafer. 
     
     
       13. The pyrometer of claim 10, wherein said means for collecting light includes: a bandpass filter for passing infrared light centered around 1.46 microns in wavelength, and   a germanium photodiode for sensing the amount of the light passed by said filter.   
     
     
       14. The pyrometer of claim 10, wherein said means for illuminating said heated body includes a halogen lamp. 
     
     
       15. In a pyrometer device for measuring semiconductor wafer temperature during fabrication processes based on the Stefan-Boltzmann and Planck's radiation formulae in accordance with the amount of light radiated by the wafer, the improvement comprising: means for intermittently illuminating said wafer with infrared light at a first frequency;   means for collecting light from said wafer at a second frequency;   means for measuring the amount of said light that is reflected by said wafer in a band centered between 5. 0 and 0.8 microns in wavelength when the wafer is illuminated as a function of the collected light and calculating the emittance of said wafer based on said measurement; and   means for deriving a temperature signal based on the calculated value of said emittance and on the amount of light collected when the wafer is not illuminated.   
     
     
       16. The device of claim 15, wherein said means for illuminating said wafer includes: a halogen lamp, and   a fiber optic means for conducting light from said lamp to said body.   
     
     
       17. The device of claim 15, wherein said means for measuring reflected light includes: a bandpass filter for passing infrared light centered around 1.46 microns in wavelength, and   a germanium photodiode for sensing the amount of light passed by said filter.   
     
     
       18. The device of claim 15, wherein said means for measuring reflected light includes: a bandpass filter for passing infrared light centered around 1.90 microns in wavelength, and   a germanium photodiode for sensing the amount of light passed by said filter.   
     
     
       19. Improved apparatus for processing a semiconductor wafer comprising source means for flooding the wafer with radiant energy for heating the wafer, the source means including cooling means that attenuates energy within an infrared range of wavelengths, and wherein the improvement comprises a pyrometer device for measuring the temperature of the semiconductor wafer, the pyrometer device comprising: means for intermittently illuminating the wafer with infrared light at a first frequency;   means for intermittently collecting light radiated and reflected by said wafer in a band centered between 5.0 and 0.8 microns in wavelength at a second frequency and for separately measuring the amount of light collected when the wafer is illuminated and the amount of light collected when the wafer is not illuminated; and   means for calculating the emittance of the wafer based on the amount of light collected when the wafer is illuminated and on the amount of light collected when the wafer is not illuminated and for deriving the temperature of the wafer based on the calculated value of the emittance and on the amount of light collected when the wafer is not illuminated.   
     
     
       20. Apparatus according to claim 19, wherein the flood means comprises an argon lamp and the cooling means comprises a water jacket that surrounds the lamp. 
     
     
       21. Apparatus according to claim 19, wherein the means for intermittently illuminating the wafer includes a halogen lamp and the means for collecting light radiated and reflected by the wafer includes a bandpass filter passing infrared light at a wavelength within said range of wavelengths attenuated by the cooling means. 
     
     
       22. An improved pyrometer of the type having means for intermittently illuminating a heated body with light, means for detecting light from the heated body within a predetermined band of wavelengths to generate an electrical signal, means for determining a temperature for the heated body from the electrical signal, and means for displaying the temperature, the improvement comprising means for intermittently collecting the light from the heated body so that the electrical signal represents first intervals when light from the heated body is obscured from the detecting means, second intervals when only light radiated from the heated body is passed by the detecting means, and third intervals when both radiated and reflected light from the heated body is passed by the detecting means, the determining means measuring from the first intervals a luminous flux value, from the second intervals a radiated flux value and from the third intervals a reflected plus radiated flux value and calculating from the luminous flux value, the radiated flux value and the reflected plus radiated flux value an emittance for the heated body and from the emittance and the radiated flux value a temperature for the heated body for display by the displaying means. 
     
     
       23. An improved pyrometer as recited in claim 22 wherein the intermittently illuminating and collecting means comprise: a disk rotatably mounted between a light source of the illuminating means and the detecting means, the disk having an inner set of apertures and an outer set of apertures located at different radial distances from the center of rotation and circumferentially spaced so that all apertures within each set are equally spaced from each other, the number of apertures in the inner set being different from the number of apertures in the outer set, the light from the light source being guided through one set to illuminate the heated body, and the detecting means being situated to collect light from the heated body that passes through the other set; and   means for generating a timing signal for each rotation of the disk, the timing signal being used by the determining means to measure the respective flux values.